The two major causes of retinal photoreceptor cell degeneration are age-related macular degeneration (ARMD) and retinitis pigmentosa. ARMD usually occurs in older individuals, and is characterized by a degeneration of the retinal pigment epithelium whereby photoreceptor cells of the central retina degenerate. Retinitis pigmentosa represents a group of human hereditary retinal degenerations which are named for the characteristic intraretinal pigment which appears around the mid-peripheral retina of individuals with retinitis pigmentosa (Berson, E. L. (1996) PNAS 93:4526-4528). Retinitis pigmentosa is a disorder which has been linked to a number of different genes. The condition primarily affects the rod cells of the retina, but can eventually lead to loss of peripheral vision and blindness. Both retinitis pigmentosa and ARMD primarily involve the degeneration of retinal photoreceptor cells, while other neurons of the retina, including retinal ganglion cells whose long axons form the optic nerve, are substantially preserved.
Both ARMD and retinitis pigmentosa affect a substantial portion of the population. ARMD affects approximately 15,000,000 people in the United States. It is estimated that total blindness eventually results in 5-10% of these persons. ARMD accounts for 17% of new cases of blindness in the United States annually. Because ARMD is primarily a disease of individuals over 65 years old, the incidence of ARMD is expected to increase as the population in the United States ages. There are approximately 100,000 individuals affected with retinitis pigmentosa in the United States, which unlike ARMD, affects younger and older individuals. The number of persons diagnosed with retinitis pigmentosa who eventually progress to complete blindness is difficult to estimate, because many retinitis pigmentosa patients develop ‘tunnel vision” with a small island of preserved vision. If approximately 5% of ARMD patients and 50% of retinitis pigmentosa patients become blind from their disease, an estimated total of 800,000 patients would be candidates for treatment for degenerative photoreceptor cells.
One possible method for restoring vision in patients afflicted with a photoreceptor degeneration disorder is through the use of a visual input which is capable of activating retinal ganglion cells. If activation by the visual input is successful, then varying degrees of vision should be restored. One suggested method for activating retinal ganglion cells through a visual input is with an electronic stimulator which electrically activates the retinal ganglion cells. Known as a “retina chip” or “silicon retina,” an electronic stimulator is the focus of much research and development, however there remain many problems associated with the use of an electronic stimulator. For example, spatial resolution problems associated with the stimulating array may occur, as well as toxicity which often accompanies the use of a foreign object in the body. Due to these complications, use of electronic stimulators is still years away from clinical application. There thus remains a need for a treatment for patients suffering from degeneration of their retinal photoreceptor cells.